1. Field of the Invention
The present invention relates to an ink jet print head of the type in which pressure generating chambers communicate with nozzle openings, each pressure generating chamber includes an elastic film and a piezoelectric element formed on the elastic film, and the piezoelectric element is displaced to eject ink droplets through the nozzle opening.
2. Discussion of the Prior Art
There is known an ink jet print head of the type in which pressure generating chambers communicate with nozzle openings, each pressure generating chamber includes an elastic film and a piezoelectric element formed on the elastic film, and the piezoelectric element is displaced to pressurize ink within the pressure generating chamber to cause the chamber to eject ink droplet or droplets through its associated nozzle opening. The ink jet print head, currently marketed, is classified into two types of ink jet print head: a first type of ink jet print head constructed by the utilization of a piezoelectric actuator which vibrates in a longitudinal direction, viz., it expands and contracts in the axial direction of the piezoelectric element, and a second type of ink jet print head by the utilization of a piezoelectric actuator in a flexural vibration aspect.
In the first type of ink jet print head, the volume of the pressure generating chamber is varied by bring the end face of the piezoelectric element into contact with the elastic film. This type of ink jet print head is suitable for a high density printing. However, its manufacturing process is complicated since the following manufacturing steps, technically difficult and additional, are required: to cut the piezoelectric element at the pitches of the array of nozzle openings so as to have a saw-tooth shape, and to position and fasten the thus cut piezoelectric element to the pressure generating chamber.
In the second type of ink jet print head, the piezoelectric element may be attached to the elastic film in a relatively simple manner: a green sheet of piezoelectric material is stuck onto the pressure generating chambers after the patterning of the pressure generating chambers, and the resultant structure is sintered. This type of ink jet print head utilizes a flexure vibration. Therefore, a relatively large area is required for producing the print head. This fact makes it difficult to form the print head of a density array.
To solve the above problems, another print head is proposed in Japanese Patent Laid-Open Publication No. Hei-5-286131. In the publication, a piezoelectric element is formed uniformly over the entire surface of an elastic film by film forming technique. The piezoelectric layer is separated after the patterning of pressure generating chambers by a lithography method. The piezoelectric elements are formed one for one pressure generating chamber.
The technique of the publication succeeds in eliminating the work to stick the piezoelectric elements onto the elastic films, and it allows the piezoelectric actuator to be stuck onto the pressure generating chamber by the precise and simple process, or the lithography method. Further, the technique has other advantages: only the piezoelectric actuators are thinned, and hence the resultant print head is operable at high speed. In this case, the piezoelectric actuators associated with the pressure generating chambers can be driven in a state that the piezoelectric layer is layered over the entire surface of the elastic film, and at least the upper electrodes are provided one for each pressure generating chamber. The piezoelectric active portions, each consisting of the piezoelectric layer and the upper electrode layer, are each preferably confined within the region on its associated pressure generating chamber, when considering a quantity of displacement of the piezoelectric actuator for unit drive voltage, a stress acting on the piezoelectric layer at a bridge between the region facing the pressure generating chamber and a region other than the former.
Generally, a piezoelectric constant of a piezoelectric thin film is 1/2 to 1/3 as large as of a piezoelectric thick film. Therefore, the use of the piezoelectric thin film fails to provide of the ejection of an effective amount of ink.
To increase the quantity of displacement of the piezoelectric actuator when it is driven, it is desirable to increase its compliance by thinning the lower electrode. In this case, however, another problem arises which reduces a pressurizing force to be additionally applied.
If the lower electrode is patterned leaving only its portion corresponding to the piezoelectric active portions, the quantity of vibrator displacement is increased retaining an optimum compliance. In this case, the wiring pattern serving also as lower electrodes cannot be secured. Attempt to realize both ends entails the increase of the number of patterning steps and cost to manufacture.